Pneumatically actuated telescoping masts are well known in the prior art and they are generally of such a nature that they may be mounted on the roof of a motor vehicle, such as an emergency vehicle or utility vehicle. In such applications, the mast is generally used for positioning electrical devices, particularly lighting fixtures, at an elevated point above the vehicle. The effect is to immediately light a large area above the vehicle to allow emergency procedures to be performed under the light, such as at accident scenes or by utility work crews such as after a storm. Pneumatically actuated telescoping masts are particularly advantageous in such situations because they are lightweight, compact in a retracted position and generally have fall, open and unobstructed mast sections which permit electrical wiring to pass within and through the telescopic tubular mast sections for controlling electrical devices or fixtures.
Typically, the telescoping mast is in a horizontal position when the mast is in a stored, retracted position on the vehicle. The mast construction is typically rotated to a vertical upright position whereat it is extended by adjacent mast sections telescoping out from one another. However, as discussed in the '837 patent, there are emergency situations where the mast is desired to be raised at some angle relative to horizontal which is less than a vertically upright position. Further, there can be some applications where the mast is desired to be moved to the side of the vehicle and then raised.
The mounting arrangement typically used for securing the mast to the vehicle generally comprises a base plate which is secured to the vehicle and from which vertically extends a pair of transversely spaced, support plates which receives the lowest mast section therebetween. A pivot shaft or a pair of stubs shafts secured near to the bottom of the lowest mast section is journalled within the shaft support plates. Various lift mechanisms have been employed to rotate the telescoping mast construction about the pivot shaft from the horizontal to the vertical positions. All of these lift mechanisms are conceptually sound and adequately function to properly position the telescoping mast construction. However, the prior art lift mechanisms do not achieve the objectives of the present invention.
In the '837 patent, a toothed bell crank attached to the swivel shaft cooperates with a rack pinion drive at one side of the support shaft to rotate the mast construction to any desired vertical angle. There are some disadvantages to having an exposed tooth/rack drive arrangement and the position of the swivel produces a higher vertical mount arrangement than desired for some vehicular applications.
The '635 patent utilizes a mechanical actuator rod mechanism to avoid the exposed tooth/gear arrangement disclosed in the '837 patent but positions the pivot shaft at the center of the lowest mast section similar to that disclosed in the '837 patent. This results in the telescoping mast construction having a horizontal stored position vertically spaced from the vehicle's roof which is higher than what is otherwise possible.
In the '451 patent, the stub shafts are journalled at a laterally offset axis relative to longitudinal centerline of the lowest mast section. However, a hydraulic actuator is utilized and an intermediate link is positioned between the stub shaft bell crank and the hydraulic cylinders actuator rod which is not a particularly advantageous arrangement.
An extremely robust, sound lift arrangement the assignee commercially utilizes for its mast constructions is diagrammatically illustrated in FIGS. 1A and 1B. As shown, the lowest mast section 1 has secured thereto pivot shaft 2 rotatable about an axis laterally spaced from the longitudinal centerline 3 of lowest mast section 1. Mounted to the bottom end of lowest mast section 1 is a lift bracket 5 secured to the bottom of lowest mast section 1. An actuator rod 6 pinned to the bottom of lift bracket 5 rotates the mast construction from horizontal to vertical positions set by angular limit switches such as illustrated in the '635 patent. While positioning the lift bracket between the shaft support plates produces a lift arrangement which (unlike the '837, the '451 and the '635 patents) does not produce any bending moment on the lift mechanism, the actuator must be positioned beneath lowest mast section 1 resulting in a higher elevation mount designated by reference arrow E than what may be desirable for certain vehicular applications.